This invention relates to a modulating method and a demodulating apparatus and, more particularly, to a modulating method for modulating data so as to be suited to transmission or recording of data on a recording medium and a demodulation apparatus for reproducing the data by demodulating the modulated coded data resulting from the demodulation.
When transmitting the data or recording the data on the recording medium, such as a magneto-optical disc or an optical disc, the data are modulated so as to be suited for transmission or recording. Block coding has been known as one of such modulating techniques. The block coding consists of blocking an input data string into units (data words) each consisting of m.times.1 bits and translating the data words into codewords, each consisting of n.times.1 bits, in accordance with an appropriate coding rule. The block coding becomes a fixed length code if i=1 and becomes a variable length code if i may assume multiple values, that is if i&gt;1, with the maximum value of i being i.sub.max =r). The code produced by block coding is termed the variable length code (d, k; m, n; r), where i is termed a constraint length, with i.sub.max being equal to the maximum constraint length r, d is the minimum number of the same consecutive symbols, that is a so-called minimum run of O's, and k is the maximum number of the same consecutive symbols, that is a so-called maximum run of O's.
When recording the variable length code on, for example, an optical disc, it is modulated into so-called non-return to zero code (NRZ code) and recorded based upon the NRZ modulated variable length code, referred to hereinafter as the recording waveform data string. With the minimum length between transitions and the maximum length between transitions of the recording waveform data string being T.sub.min and T.sub.max, respectively, it is desirable from the viewpoint of the recording density that the minimum length between transitions T.sub.min be long, that is that the minimum run d be large, while it is desirable from the viewpoint of clock reproduction and jitter that the maximum length between transitions T.sub.max be short, that is that the maximum run k be small. A variety of modulation methods have been proposed with which the above conditions are met.
In the case of an optical disc having audio data recorded thereon, that is a so-called compact disc (CD), a so-called eight-to-fourteen modulation (EFM) is employed. This modulation method is a variable length code (2, 10; 8, 17; 1) in which, if the interval between bits (bit length) of the recording waveform data string is T, the minimum length, between transitions T.sub.min is (2+1)T or 3T. If the interval between bits of the data string is T1, the minimum length between transitions T.sub.1min is ((8/17).times.3)T.sub.1 or 1.41T.sub.1. In the following description, the indication of the bit length T is directly followed by the bracketed indication of the corresponding bit length T.sub.1. The maximum length between transitions T.sub.max (T.sub.1max) is 11T (5.18T.sub.1,), while the window margin T.sub.W, which stands for the allowable jitter, is represented as (m/n).times.T, and has a value equal to (8/17)T or 0.47T. With such a CD, it may be contemplated to diminish the minimum pit length corresponding to the minimum length between transitions T.sub.min (T.sub.1max) on the optical disc. However, if the minimum pit length becomes excessively shorter than the spot size of the laser beam, it becomes difficult to detect the pits so that errors tend to be produced. On the other hand, limitations are imposed on increasing the recording density by diminishing the wavelength of the laser light source for diminishing the spot size.
With the above in view, there has also been developed a modulation method whereby the minimum pit length on the optical disc, that is the minimum length between transitions T.sub.min (T.sub.1min), may be increased without diminishing the information volume.
For example, as shown in table 1, there has also been proposed a variable length code (4, 22; 2, 5; 5) in which the minimum length between transitions T.sub.min is increased to 5T (2T.sub.1) for increasing the recording density as compared to EFM. This modulation method is referred to hereinafter as variable five modulation (VFM). With the VFM, the maximum length between transitions T.sub.max (T.sub.1max) increased to 23T (9.2T.sub.1,) as compared to the maximum length between transitions T.sub.max (T.sub.1max) of EFM which is equal to 11T (5.18T.sub.1,). That is, the VFM is not advantageous from the viewpoint of clock reproduction and jitter and renders the designing of the apparatus difficult.
TABLE 1a ______________________________________ data words code words ______________________________________ i=1 11 00000 10 10000 111111 00001 00001 00000 ii=2 0111 01000 00000 0110 00100 00000 0101 00010 00000 0100 00001 00000 i=3 001111 01000 01000 00000 001110 01000 00100 00000 001101 01000 00010 00000 001100 01000 00001 00000 001011 00010 00001 00000 001010 00100 00100 00000 001001 00100 00010 00000 001000 00100 00001 00000 000111 00010 00010 00000 i=4 00011011 01000 01000 01000 00000 00011010 01000 01000 00100 00000 00011001 01000 01000 00010 00000 00011000 01000 01000 00001 00000 00010111 01000 00010 00001 00000 00010110 01000 00100 00100 00000 00010101 01000 00100 00010 00000 00010100 01000 00100 00001 00000 00010011 01000 00010 00010 00000 00010010 00100 00100 00100 00000 00010001 00100 00100 00010 00000 00010000 00100 00100 00001 00000 00001111 00010 00001 00001 00000 00001110 00100 00001 00001 00000 00001101 00100 00010 00010 00000 00001100 00100 00010 00001 00000 00001011 01000 00001 00001 00000 00001010 00001 00001 00001 00000 00001001 00010 00010 00010 00000 00001000 00010 00010 00001 00000 ______________________________________
TABLE 1b ______________________________________ data words code words ______________________________________ i=5 0000011111 01000 01000 01000 01000 00000 0000011110 01000 01000 01000 00100 00000 0000011101 01000 01000 01000 00010 00000 0000011100 01000 01000 01000 00001 00000 0000011011 01000 01000 00010 00001 00000 0000011010 01000 01000 00100 00100 00000 0000011001 01000 01000 00100 00010 00000 0000011000 01000 01000 00100 00001 00000 0000010111 01000 01000 00010 00010 00000 0000010110 01000 00100 00100 00100 00000 0000010101 01000 00100 00100 00010 00000 0000010100 01000 00100 00100 00001 00000 0000010011 01000 00010 00001 00001 00000 0000010010 01000 00100 00001 00001 00000 0000010001 01000 00100 00010 00010 00000 0000010100 01000 00100 00010 00001 00000 0000001111 01000 01000 00001 00001 00000 0000001110 01000 00001 00001 00001 00000 0000001101 01000 00010 00010 00010 00000 0000001100 01000 00010 00010 00001 00000 0000001011 00100 00100 00010 00010 00000 0000001010 00100 00100 00100 00100 00100 0000001001 00100 00100 00100 00010 00000 0000001000 00100 00100 00100 00001 00000 0000000111 00100 00100 00010 00001 00000 0000000110 00100 00100 00001 00001 00100 0000000101 00100 00010 00010 00010 00000 0000000100 00100 00010 00010 00001 00000 0000000011 00100 00100 00010 00001 00000 0000000010 00100 00100 00001 00001 00000 0000000001 00100 00010 00010 00010 00000 0000000000 00100 00010 00010 00001 00000 00010 00001 00001 00001 00000 00001 00001 00001 00001 00000 SYNC for mod2to4d5 ASYNC 23T 21T 6T BSYNC 21T 23T 6T ______________________________________
In other words, although it is necessary to increase the minimum length between transitions T.sub.min for increasing the recording density, it is necessary to reduce the maximum length between transitions T.sub.max simultaneously to as small a value as possible in order to assure the operating reliability of the apparatus. That is, a smaller value of the ratio of the maximum length between transitions to the minimum length between transitions is desirable. Specifically, the ratio T.sub.max /T.sub.min =11T/3T=3.67 for EFM, while the same ratio for VFM is 23T/5T=4.60.
In sum, while it is necessary to increase the minimum length between transitions T.sub.min (T.sub.1min), that is the minimum run d, of the variable length code produced on modulation in order to increase the recording density for a recording medium, such as an optical disc, the maximum length between transitions T.sub.max (T.sub.1max), that is the maximum run k, is undesirably increased with the conventional modulation methods.